1. Field of the Invention
The present invention relates to phase-lock loop circuits and more specifically to a digital phase lock loop circuit useful for measuring Doppler frequency shift.
2. Description of the Prior Art
Analog phase-locked loop circuits have found wide application in frequency measurement, frequency synthesizers, and high sensitivity tracking receivers. However, analog systems suffer from sensitivity to DC drifts and component saturation, and a need for calibration and periodic adjustments. Digital systems do not suffer from these difficulties and in addition permit real time processing of signals. An analog phase-lock loop consists of three major functional units: (1) a phase detector, (2) a loop filter, and (3) a voltage-controlled oscillator (VCO). A block diagram of a conventional analog phase-locked loop is shown in FIG. 1. The incoming signal whose phase or frequency is to be tracked is mixed with a locally generated sine wave generated by the voltage-controlled oscillator to produce a phase error voltage proportional to the instantaneous phase difference. This error voltage is filtered and used to control the instantaneous frequency of the voltage controlled oscillator. In realizing such a system with analog circuitry, the phase detector is generally a balanced mixer, the loop filter may be a simple RC low-pass circuit, and the VCO may be tuned by adjusting the bias voltage on a varicap or other element in the tuned circuit controlling the resonant frequency.
Early efforts at replacing the analog components with digital ones are reported by William C. Lindsey and Chak Ming Chie in A Survey of Digital Phase-Locked Loops, Proc. IEEE vol. 69, pp. 410-431, April 1981. However, systems disclosed therein simply replaced the analog components with equivalent digital ones.
Th present invention provides an improved digital phase-lock loop circuit, capable of providing essentially instantaneous frequency measurements, which provides high resolution, valid data over short received intervals, and is capable of locking within one cycle of the reference frequency source, since there is no filtering of the error signal.